It has been known in the past to arrange a nitrogen oxide sensor (NOX sensor) in an exhaust passage of an internal combustion engine so as to detect a concentration of nitrogen oxide (NOX) in exhaust gas flowing through the exhaust passage of the internal combustion engine.
As described in PLT 1, an NOX sensor is provided with a measured gas chamber into which exhaust gas is introduced as measured gas, a pump cell discharging oxygen in the measured gas, and a sensor cell detecting a concentration of NOX in the measured gas. A negative electrode of the pump cell is comprised of a platinum-gold alloy (Pt—Au alloy) having an oxygen decomposition function and not having an NOX decomposition function. On the other hand, a negative electrode of the sensor cell is comprised of a platinum-rhodium alloy (Pt—Rh alloy) having an NOX decomposition function.
Further, the NOX sensor is also provided with a heater for heating the sensor cell and pump cell. The heater heats the sensor cell and pump cell to the activation temperatures or more so as to secure the precision of detection of the NOX concentration by the NOX sensor. However, if the temperature of the pump cell becomes excessively high, gold (Au) will evaporate from the negative electrode of the pump cell. The evaporated Au will deposit on the negative electrode of the sensor cell and cause the output of the sensor cell to permanently drop. For this reason, while the NOX sensor detects the concentration of NOX in the exhaust gas, the temperature of the pump cell is controlled to a predetermined control temperature (for example 750° C. to 800° C.)